The invention is in the area of tertiary oil recovery techniques, in particular, an improved apparatus for downhole injection of steam into boreholes.
In the art of recovering oil from earth formations, tertiary methods are increasing in their importance. Initially, oil flow from many wells is driven by the pressure due to natural gases trapped along with the liquid oil in the formation. With the passage of time, natural gas pressures decrease. When gas pressure is insufficient to drive oil to the surface, pumping methods are then employed. As time passes, pumping methods may be ineffective because the flow of oil underground out of porous formations into a well may be very slow. It is at this point that tertiary methods are sought to accelerate the flow of oil from the formation into the well.
A particularly useful tertiary method employs the injection of steam. Steam serves to heat the oil in the formation, thereby reducing its viscosity and increasing its flow rate into the well for recovery.
Methods employing downhole generation of steam within a well have proved to be particularly advantageous. The prior art discloses several representative methods and apparatus.
In U.S. Pat. No. 3,456,721, Smith discloses a downhole burner for generating steam. Gaseous or liquid fuels are mixed with air and combusted in a burner with simultaneous spraying of water toward the flame. The water is sprayed from a cylindrical water jacket through a plurality of orifices. Steam is formed by the vaporization of the water as the water bombards the flame.
In U.S. Pat. No. 3,980,137, Gray discloses a downhole steam injector employing the combustion of hydrogen with oxygen to generate heat to vaporize injected water to form steam. The water moves through an annular preheater jacket surrounding the combustion chamber and, after being preheated, enters the combustion chamber through a plurality of grooves or passages at the top of the combustion chamber near the igniter and the hydrogen/oxygen flame.
Hamrick et al in their related U.S. Pat. Nos. 3,982,591 and 4,078,613 disclose downhole steam generators. In the first patent, in FIG. 17, water is injected through a plurality of apertures directly into the flame in a hydrogen/oxygen combustion zone. In the second patent, in FIG. 2B, water moves through a cooling annulus before it is injected into a mixing zone spaced below the combustion zone. The mixing zone is defined by a cylindrical wall which has a plurality of apertures through which water from the cooling annulus passes laterally into the mixing zone. A heat resistant liner is placed along the interior of the combustion zone.
Several problems have been encountered with these prior art downhole steam generators. One problem is the tendency of the flame in the combustion chamber to be quenched by water injected directly into the flame to produce steam.
An additional problem related to the direct injection of water into the combustion zone is wetting the sparkplug or igniter, thereby reducing the efficiency of combustion.
A problem related to inefficient combustion is the formation of soot. Soot has two deleterious effects. One is air pollution, and the other is the tendency to clog the pores in the earth formation, thereby impeding oil flow out of the formation into the production well.
Another problem relates to the products of combustion which are acidic gases such as carbon dioxide, sulfur oxides, and nitrogen oxides. In certain types of earth formations, it is undesirable to contact the formation with the acidic products of combustion.
Another undesirable aspect of prior art downhole steam generators is the necessity for supplying both fuel and oxidant downhole at relatively high pressures. Expensive high pressure compressors are required for the oxidant, namely air. Expensive high pressure pumps are required for the fuel, namely a liquid hydrocarbon such as diesel fuel, fuel oil or kerosene.
Problems are also encountered relative to the efficient preheating of the fuels and water used in the downhole steam generator. To explain, liquid fuels may be relatively cold at the surface prior to pumping downhole. As a result, the combustion process itself, must give up heat to the liquid fuel to bring it up to combustion temperatures. Cool fuel, results in production of soot, which is undesirable because of surface air pollution or clogging of pores in the earth formation. Similarly, water may be relatively cold at the surface prior to pumping downhole. As a result, a considerable portion of the heat generated by the combustion process is consumed in bringing the water up to the boiling point. Thus, less energy is available for production of high enthalpy steam.
With prior generators, conditions downhole may occur which tend to flood the combustion zone with reservoir fluids. This occurs particularly when a temporary interruption of combustion is encountered. A need for isolating the combustion chamber from reservoir fluids is thus indicated.